Numerous products of commercial significance are formed of elastomeric compositions wherein particulate filler is dispersed in any of various synthetic elastomers, natural rubber or elastomer blends. Carbon black, for example, is widely used as a reinforcing agent in natural rubber and other elastomers. It is common to produce a masterbatch, that is, elastomer coagulated with carbon black or other filler and optionally containing various additives, such as extender oil. Carbon black masterbatch can be prepared with different grades of carbon black, that is, carbon blacks which vary both in surface area per unit weight and in “structure.”
While a wide range of performance characteristics can be achieved employing currently available materials and manufacturing techniques, there has been a longstanding need in the industry to develop elastomeric compositions having improved properties and to reduce the cost and complexity of current manufacturing techniques. In particular, it is known for example that macro-dispersion level, that is, the uniformity of dispersion of the carbon black or other filler within the elastomer, can significantly impact performance characteristics. For elastomeric compositions prepared by intensively mixing the carbon black or other filler with natural rubber or other elastomer (such as in a Banbury mixer or the like), any increase in macro-dispersion requires longer or more intensive mixing, with the consequent disadvantages of increased energy costs, manufacturing time, and similar concerns. For carbon black fillers of certain surface area and structure characteristics, dispersion beyond a certain degree has not been possible or commercially practicable using known mixing apparatus and techniques. In addition, such prolonged or more intensive mixing degrades the natural rubber or other elastomer by reducing its molecular weight, rendering the finished elastomeric compound undesirable for certain applications.
In addition to dry mixing techniques, it is known to continuously feed latex and a carbon black slurry to an agitated coagulation tank. Such “wet” techniques are used commonly with synthetic elastomer, such as styrene-butadiene rubber (SBR). The coagulation tank contains a coagulant such as salt solution or an aqueous acid solution typically having a pH of about 2.5 to 4. The latex and carbon black slurry are mixed and then coagulated in the coagulation tank into small beads (typically a few millimeters in diameter) referred to as wet crumb. The crumb and acid (or saline) effluent are separated, typically by means of a vibrating shaker screen or the like. The crumb is then dumped into a second agitated tank where it is washed to achieve a neutral or near neutral pH. Thereafter the crumb is subjected to additional vibrating screen and drying steps and the like. Variations on this method have been suggested for the coagulation of natural and synthetic elastomers. In U.S. Pat. No. 4,029,633 to Hagopian et al, which like the present invention is assigned to Cabot Corporation, a continuous process for the preparation of elastomer masterbatch is described. An aqueous slurry of carbon black is prepared and mixed with a natural or synthetic elastomer latex. This mixture undergoes a so-called creaming operation, optionally using any of various known creaming agents. Following the creaming of the carbon black/latex mixture, it is subjected to a coagulation step. Specifically, the creamed carbon black/latex mixture is introduced as a single coherent stream into the core of a stream of coagulating liquor. The solid stream of creamed carbon black/latex mixture is said to undergo shearing and atomizing by the stream of coagulating liquor prior to coagulation, being then passed to a suitable reaction zone for completion of the coagulation. Following such coagulation step, the remainder of the process is substantially conventional, involving separation of the crumb from the waste product “serum” and washing and drying of the crumb. A somewhat similar process is described in U.S. Pat. No. 3,048,559 to Heller et al. An aqueous slurry of carbon black is continuously blended with a stream of natural or synthetic elastomer latex. The two streams are mixed under conditions described as involving violent hydraulic turbulence and impact. As in the case of the Hagopian et al. patent mentioned above, the combined stream of carbon black slurry and elastomer latex is subsequently coagulated by the addition of an acid or salt coagulant solution.
Since good dispersion of a coagulating filler in the elastomer has been recognized for some time as being important for achieving good quality and consistent product performance, considerable effort has been devoted to the development of procedures for assessing dispersion quality in rubber. Methods developed include, e.g. the Cabot Dispersion Chart and various image analysis procedures. Dispersion quality can be defined as the state of mixing achieved. An ideal dispersion of carbon black is the state in which the carbon black agglomerates (or pellets) are broken down into aggregates (accomplished by dispersive mixing) uniformly separated from each other in the elastomer (accomplished by distributive mixing), with the surfaces of all the carbon black aggregates completely wetted by the rubber matrix (usually referred to as incorporation).
Macro-dispersion of carbon black or other filler in uncured natural rubber or other suitable elastomer can be assessed using image analysis of cut surface samples. Typically, five to ten arbitrarily selected optical images are taken of the cut surface for image analysis. Knife marks and the like preferably are removed using a numerical filtering technique. Cut surface image analysis thus provides information regarding the carbon black dispersion quality inside a natural rubber compound. Specifically, percent undispersed area D(%) indicates carbon black macro-dispersion quality. As macro-dispersion quality is degraded, percent undispersed area increases. Dispersion quality can be improved, therefore, by reducing the percent undispersed area.
A commercial image analyzer such as the IBAS Compact model image analyzer available from Kontron Electronik GmbH (Munich, Germany) can be used to measure macro-dispersion of carbon black or other filler. Typically, in quantitative macro-dispersion tests used in the rubber industry, the critical cut-off size is 10 microns. Defects larger than about 10 microns in size typically consist of undispersed carbon black or other filler, as well as any grit or other contaminants, which can affect both visual and functional performance. Thus, measuring macro-dispersion involves measuring defects on a surface (generated by microtoming, extrusion or cutting) greater than 10 microns in size, by total area of such defects per unit area examined, using an image analysis procedure. Macro-dispersion D(%) is calculated as follows:
      Undispersed    ⁢                  ⁢          area      ⁡              (        %        )              =            1              A        m              ⁢                  ∑                  i          =          1                m            ⁢                          ⁢                        N          i                ⁢                              Π            ⁢                                                  ⁢                          D              i              2                                4                    where                Am=Total sample surface area examined        Ni=Number of defects with size Di         Di=Diameter of circle having the same area as that of the defect (equivalent circle diameter)        m=number of images        
There has long been a need in various industries for elastomeric compounds of particulate filler dispersed in suitable elastomer, especially, for example, carbon black dispersed in natural rubber, having improved macro-dispersion. As discussed above, improved macro-dispersion can provide correspondingly improved aesthetic and functional characteristics. Especially desirable are new elastomeric compounds of carbon black in natural rubber wherein improved macro-dispersion is achieved together with controlled Mooney Viscosity, higher molecular weight of the natural rubber, and higher amount of bound rubber.
It is an object of the present invention to meet some or all of these long felt needs.